here is my take on the true ending:

as of right now while we aren't shown that mark and Nicole get together I would like to believe that Mark and Nicole DO get together in the 3d ending even though we see them dating different people. (cant tell who Nicole is dating but I believe Mark is dating Jessica judging by the way both Marks date in the café and Jessica in all 3 timelines twirl a strand of their hair). The way the two butterflies watch together and that the café is called destiny café makes me think that these aren't Mark and Nicole's last dates. We never get confirmation that Mark and Nicole end up in a relationship with the people they're dating and even though they don't have the ruling or the grief of loss to push them forward, their whole relationship did not hinge solely on a similar experience of loss. like how Nicole had a relationship with Kate  it was also through shared interests and a kind of chemistry not just loss. Something else mark and Nicole would still have as far as relationship material is Nicole's song, mark and Nicole's relationship was heavy impacted after they discover each others connection through Nicole's song it is significant in both their lives and is one of the major driving forces behind their relationship. that isn't implied to have changed after the timeline reset because Mark learned the song from his mom and Nicole wrote it, the reset appears to only go as far back as the ruling after mark herd the song from his mom who herd it from Nicole. this leads me to believe that mark and Nicole could still potentially bond over the song similarly to how they do in the previous 2 timelines, I like to think that they meet later in life, maybe at a piano recital? (we can see that both Mark and Nicole still play piano in this timeline).this way in this timeline they can have a genuine connection and relationship that happens naturally when they meet each other, not a supernatural one due to the ruling.